No pressure on front brakes, mc piston failed? SORTED

[inazumin]

Hi all.

I have just replaced the piston seals on both front two pot Suzuki sliding calipers (bandit 600 type), and after bleeding the fluid I have no pressure. This is something I have done many times in the past 30 years without a problem.

Each calliper was done separately clamping the pipe, then refilling caliper as best I could before refitting brake line banjo. What happens now is when the lever is pulled, as I hear the ‘stoplight’ switch click the pads clamp the discs, but I then have to pull the lever back another 10-15mm before the lever then comes solid. When held at this point the lever will then move back to within 10mm of the twist grip.

All joints checked for leaks, lever tied back to bar overnight, rock solid next morning, but a few squeezes on the lever and its back to the same. Pistons not stuck, both pairs move out together as far as I dare with pads removed. 2 ltrs of fluid now used, have done back bleeding from caliper to mc, and pulled thru with syringe pump. All to no avail.

Hover if I push the pistons all the way into the callipers and support a file between the disc and pad to keep pistons in it all works fine – two finger solid at ‘stoplight’ click.

If I clamp one hose or the other then it is solid, local dealer said this is way to test mc piston. so assuming is ok.

Everything was Ok before I replaced seals due to one leaking.
Any ideas truly appreciated

[stinkwheel]

Try bleeding it at the top banjo bolt. Wrap a cloth round the master cylinder to soak up the fluid and crack the bolt while squeezing the lever like you would the bleed nipple.

Sometimes get a stubborn air bubble there.
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I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[Kickstart]

Hi

I assume that the calipers are sliding freely.

Wrap some cloth around the top banjo bolt, pull the lever and then crack off and tighten the banjo bolt, then release the lever. Air can get caught in the banjo bolt and this helps to get rid of it (the cloth is just there to catch the fluid that squirts out).

Also try angling the bars so the hole from the master cylinder to th reservior is the higher than the system after it and pull the lever gently. With luck as you gently pull the lever air will come from the master cylinder into the reserviour.

All the best

Keith
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[inazumin]

Ah thanks, did wonder, but as bike is ten years old was worried about the washers resealing after, will pluck up courage and have a go when I get home.

edit; calipers sliding fine, did full over haul.

[Kickstart]

Hi

Possible that a seal was damaged when assembled, but hopefully not.

Had a set on a ZZR600 that would not bleed through at all. If you pumped like crazy you could get a firm lever, but release it for 5 seconds or so and it was back to being soft.

All the best

Keith
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[inazumin]

[Kickstart]

Hi

Maybe, but not always. I clipped a seal and took a small chunk out of it on my TZR250 a couple of months ago. No leakage, but no pressure either.

All the best

Keith
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[inazumin]

[sickpup]

Which year bandit is it?

Genuine or Pattern seals?

[inazumin]

[inazumin]

Right, odd things going on, works when calipers off discs and wedged with steel between pads. so going to start again by getting pistons out.

Any other way of double checking mc piston/seal is okay.

dave

[Kickstart]

Hi

Can you double check that nothing strange is happening with how the calipers float. Ie, disk offset so the caliper cannot float far enough and so bending the disk, "suction" pulling the caliper back too far, etc.

All the best

Keith
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[Blue_SV650S]

Try taking the callipers off and resting them on the discs i.e. remove the calliper bolts, but leave caliper on disc. Does all work ok (we know it does when pistons all the way in, but does it with pistons out, but calliper free?

I have had this before, it was basically a calliper mount on the fork that was bent (presumably from a crash or disc lock accident!! ).

To rectify, I 'shimmed' out the calliper mount ... a lot of faffing with offcuts of metal, but it worked!!
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[Thom]

inazumin i had the exact same problem on the sliding caliper of my diversion 6. i tried taking the caliper off the bike and doing as you described and it worked fine until it was on the bike.

what i found was the sliding action of the caliper was absorbing alot of the pressure coming from the fluid, i traced the problem back to the MC as the caliper was obviously ok. had the MC apparat and found loads of crud around the mc seals and parts, cleaned it all out, reassembled and it bled as normal.
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Current Bike: 1996 Yamaha TRX850 (The Japanese-Ducati One).
Previous Bikes: 1990 Honda NSR125 'Rothmans' (The smoking one), 1990 Suzuki GS500e 'Caf? fighter' (The loud one), 1987 Kawasaki GPX400r (The quick one), 1997 XJ600s Diversion (The reliable one), 2000 Kawasaki ER-5 (The spontaneously combustive one)

[inazumin]

[Thom]

its funny your describing exactly what happened on my bike also, i made a theory that basically went liek this:

Youve had no fluid in the lines for some reason, so you reassemble and begin to bleed, their is no hydraluic action to pull fluid through the system so youve got to wait for gravity to do it, once your caliper is finally full there is only so much fluid in the caliper for this reason alone, and likely quite alot of air. So you begin to bleed it, but because you've pushed the pistons back into the caliper there isnt as much fluid in there as previously.

So the MC/piston should force more and more fluid into the lines/caliper until 'full' but because of a failure in the MC the fluid isnt remaining in the lines, meaning only so much can be in there and under pressure.

Basically what im saying is theres not enough fluid to push the pistons/pads out far enough, because theres a problem in the MC.

I reused everything and it works fine now.
____________________
Current Bike: 1996 Yamaha TRX850 (The Japanese-Ducati One).
Previous Bikes: 1990 Honda NSR125 'Rothmans' (The smoking one), 1990 Suzuki GS500e 'Caf? fighter' (The loud one), 1987 Kawasaki GPX400r (The quick one), 1997 XJ600s Diversion (The reliable one), 2000 Kawasaki ER-5 (The spontaneously combustive one)

[inazumin]

[inazumin]

[inazumin]

Well had a good going over and forks seem all ok. no sign of damage and brakes were OK before the seals were replaced, so I'm going the mc piston route as when calipers are loose the sliders don't move, but requiring more leverage at the lever when bolted up due to lack of pressure in the lines.

I will order a new piston set to be on safe side as I will need to use bike at weekend, so don't want to be stuck if old one is naff.

[Kickstart]

Hi

Suggests it is something in how it clamps to the fork that is causing the problem. If the caliper is mounted out of line with the disk (ie, bent pins, bent mounts on the forks, bent disk, etc) then this would explain it. Suspect what would happen is the disk would bend as the brake is used, and when the brake is released the disk would spring back and push the pistons back further.

All the best

Keith
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[inazumin]

[inazumin]

One other thing, the haynes manual and various other articles say the fluid seals are tapered, wide part facing out. Now bearing this in mind I examined them and could see no difference, so measured them at work before fitting using precision gauge, could see no difference even under magnifying glass. Spoke to my dealers mechanic and he said they aren't tapered put them in anyway round.

A friend suggested yesterday if they were in with the 'wide' end inwards it would make piston harder to push out.

Any thoughts on this, popped pistons last night and seals turned around but still the same.

dave

[Kickstart]

Hi

Some calipers do use tapered seals, but doesn't seem that common.

However you have found that the brake calipers work fine when they are not bolted to the forks. This suggests that there is nothing wrong with the hydraulic part of the system, whether caliper or master cylinder seals.

It still sounds to me like the calipers are for some reason not sliding smoothly on the pins.

All the best

Keith
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[Knacker]

Off topic i no but lovely bike are the Inazuma's

https://i210.photobucket.com/albums/bb259/Squeaky_86_2007/n606445283_5365046_2824.jpg

This 1 goes like shit off a shovel but the brakes are shite to. So if you find out what the problem is let me no as my brother in law (Whos bike it is) would love to know
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Current: Gsxr 600 Srad - GS125 - DT125
Previous: K1 GSXR 1000

[inazumin]